1. Technical Field
This invention generally relates to a material used in forming a passivating layer in integrated circuit chips, and more specifically relates to a method of utilizing a removable passivating polyimide coating in situations where it is desirable to place the coating, manipulate the system with the coating in place, and then later remove the coating.
2. Background Art
Polyimides are used extensively in the microelectronics industry. Typically, polyimides are formed by the thermal curing of polyimide precursors, such as polyamic acids, and are used as passivation dielectrics on semiconductor devices (i.e. silicon chips). A problem associated with passivating a reactive metal substrate with a polyimide material by itself is the known interaction between the polyimide and the reactive metal. As reported in Linde, H. G. and R. T. Gleason, "Cation Interactions with Polyamic Acids," J. POLYMER SCI. B, Vol. 26, 1485-1497 (1989), D. Y. Shih, J. VAC. SCI. TECH., A7(3), 1402-1412 (1989), and Kim et al., "Adhesion and Interface Studies Between Copper and Polyimide," J. ADHESION SCI. TECH., Vol. 1, No. 4, 331-339 (1987), copper and other reactive metals form salts that retard thermal imidization and decompose the polyimide polymer during the high temperature curing step. But see U.S. Pat. No. 4,908,086, issued to Goodrich et al., on Mar. 30, 1990 (disclosing a polyimide adhesive mixed with a finely divided conductive metal to replace a gold-silicon eutectic die attach that will not decompose, even when the temperature is ramped up to 460.degree. C.).
This decomposition was considered detrimental to the overall semiconductor device and, in fact, various inventions dealt with avoidance of the perceived problem. This was accomplished by either partially curing the polyimide precursor prior to adhering the metal layer or layering a first material onto the metal prior to depositing the polyimide precursors onto the first material. See U.S. Pat. No. 4,152,195, issued to Bahrle et al., on May 1, 1979 (disclosing the method of partially curing the polyimide precursor prior to vapor deposition of the desired metal); U.S. Pat. No. 4,423,547, issued to Farrar et al., on Jan. 3, 1984 (teaching the deposition of a silicon nitride onto a metal followed by the deposition of a thicker layer of polyimide); and U.S. Pat. No. 5,114,754, issued to Cronin et al., on May 19, 1992 (disclosing a process of applying a layer of silsesquioxane between the polyimide and the metal to passivate the metal and inhibit interaction between the metal and the polyimide precursor).
The advantage of placing a removable layer in certain applications has been perceived. For example, U.S. Pat. No. 4,783,695, issued to Eichelberger et al., on Nov. 8, 1988, discloses that in order to improve testability, it is desirable to place a layer of insulative adhesive onto a multichip integrated circuit package, test the package, and then later remove the insulative material should the multichip package fail. It will also allow for testing the layout of the multichip interconnects, and then allow the chips to be reused should that structure not prove to be optimal. Other patents disclose releasable adhesives, including U.S. Pat. No. 5,262,229, issued to Lampert et al., on Nov. 16, 1993 (disclosing a conductive releasable adhesive for electrostatic discharge controlled surfaces) and U.S. Pat. No. 5,216,043, issued to Sipinen et al., on Jun. 1, 1993 (disclosing a thermoplastic polymer combined with a transition metal salt to form an adhesive composition that will oxidatively degrade to an embrittled polymer), however, these releasable adhesives are not for use in semiconductor devices. Each of the articles and patents cited is incorporated herein by reference.
A second application is in the stacked and laminated semiconductor devices, commonly known as CUBES. In the process of manufacturing a CUBE, forty or more chips are formed into a large stack, which is then processed to form the edge wiring and connects. Subsequently, the large stack is segmented into shorter stacks of five or more chips. Presently, a non-polyimide adhesive layer such as ULTEM (as sold by the General Electric Company of Schenectady, N.Y.) or DITAC (as sold by E. I. DuPont de Nemours of Wilmington, Del.) are used to allow cleavage at the chosen time. However, these materials suffer from drawbacks including: the additional process steps and tooling necessary to place the layer; and the fact that the non-polyimides have characteristics, for example solubility and glass transition temperatures, which are dissimilar from the polyimides, and, therefore, cause stress and contamination problems during the processing of the CUBES.
Therefore, there exists a need to provide a removable adhesive layer for semiconductor devices that could undergo the testing process, and yet could be selectively and easily removed when desired. This removable adhesive layer may be utilized as a simple passivating layer for multichip semiconductor device testing. Also, a need was seen in the CUBE technology for a removable adhesive that was thermally compatible with the polyimide passivating layers, so that regular processing could be performed on a stack of CUBES and then the individual CUBES could be released for use as a single CUBE.